The invention relates to taking samples from holes. In another aspect, the invention relates to taking a sample from the sidewall of a borehole.
In the exploratory probing of the earth with boreholes, it is economically desirable to utilize as small a borehole as possible to extract the desired information. For example, it is a mathematical certainty that four times as much earth must be displaced from a 4" borehole than from a 2" borehole of the same depth. A downhole tool suitable for taking samples from the side of a small diameter borehole would thus be desirable.
To obtain information of the strata traversed by the borehole, sidewall sampling tools are lowered into the borehole to the level of the formation to be investigated and hollow core-taking projectiles are shot into the formation. The projectile and contained core are withdrawn from the formation by retrieving means affixed to the sidewall sampling tool. The sidewall sampling tool is then raised to the surface and the core removed from the projectile for analysis. However, in the past the cores were frequently lost from the projectile during the raising of the sidewall sampling tool through the borehole due to impact of the projectile or sidewall sampling tool against the wall of the borehole. Besides necessitating an additional lowering of the sampling tool into the borehole to take another sample, it has proved frequently difficult to position the sampling tool adjacent the same formation from which the previous, unrecovered sample was taken. An apparatus for reliably transporting a sample from the stratum to the surface would thus be extremely desirable.
Generally, sidewall samplers comprise a plurality of bores containing projectiles backed by propellant charges. The propellant charge is ignited by an ignition device such as an igniter cap, which is electrically actuated. Use of an igniter device in a downhole sampling gun presents a hazard to personnel on site. Additionally, constructing the downhole sampling gun so as to employ an igniter device to ignite the charge greatly complicates fabrication of the downhole gun. It would also be extremely desirable to provide a downhole gun which does not employ an igniter device such as an igniter cap and which is simple and easy to fabricate.